1. Field of the Invention
The present invention relates to an image display apparatus for displaying an image, which is formed on an image forming element such as a liquid crystal display element, to an observer via an optical system, and more particularly, to an image display apparatus suitable for a head-mount display (HMD).
2. Description of the Related Art
An image display apparatus such as a head-mount display (HMD) is desired to have presence enhanced by widening a view angle of an observed image, and to be miniaturized and lightened in general in order to be mounted on a head of a user. In particular, when a weight balance and an appearance of the entire apparatus are taken into account, it is desirable that the image display apparatus have a small thickness in a visual axis direction (front direction) of an observer.
In order to widen the view angle using a small liquid crystal panel, it is preferable to form an intermediate real image of an image displayed on the liquid crystal panel serving as an optical system for observation to observe a virtual image of the intermediate real image as an enlarged image. U.S. Pat. No. 7,019,909 proposes an HMD in which a decentering reflecting surface, in which light passes through a turn back reflecting surface and a periphery thereof, is used as an intermediate imaging optical system, a back-and-forth optical path is formed, and intermediate imaging is performed in the optical path, to thereby widen a view angle.
Further, the HMD preferably has a favorable appearance when the HMD is mounted on a head of an observer. In particular, when the observer wearing the HMD is viewed from the front, it is preferable that the entire apparatus be elongate in a horizontal direction in a case where the same projected area is used. For example, with respect to an observer 81 shown in FIG. 8A, it is more preferable to use an apparatus 83 which has a shorter vertical width and a longer horizontal width as illustrated in FIG. 8C than to use an apparatus 82 which has a longer vertical width and a shorter horizontal width as illustrated in FIG. 8B. Thus, in order to obtain the apparatus having a longer horizontal width, it is preferable to use a lateral optical system in which each eccentric direction of a plurality of decentering reflecting surfaces is not developed in a vertical direction of a face of an observer, but is developed in a horizontal direction of the face.
Japanese Patent Application Laid-Open No. 2004-341324 proposes an HMD in which a reflecting surface is arranged between an optical device for forming a reciprocating optical path formed by a turn back reflecting surface, and an image forming element, to thereby reduce the size of the entire apparatus. In Japanese Patent Application Laid-Open No. 2004-341324, the entire apparatus is reduced in size by adopting an optical system in which optical paths for light beams made incident on left and right eyes intersect with each other between the image forming element and the reflecting surface and between the reflecting surface and the optical device.
The optical system in which intermediate image is formed in the optical path to widen the view angle requires a large number of components and a long optical path length, so the size of the entire apparatus tends to become larger.
In addition, when the apparatus is structured to have a short vertical width and a longer horizontal width in order to obtain a favorable appearance of the apparatus in a case where the HMD is mounted on the head of a user, the thickness of the apparatus in the visual axis direction of the observer tends to be increased.